A BOPP film in general is constructed to have 3–5 layers depending upon its application. Generally, a BOPP film grade material requires specific characteristics to provide good processability and film properties because of the complexity of its converting process. Each layer requires specific characteristics and the core layer is the most important for the processability of the material on a tenter line. The material for the core layer is generally a 2–4 MFR homopolymer having relatively large amounts of atactic polymer, i.e., xylene solubles, and/or low isotacticity of xylene insoluble fraction. Low isotacticity of the xylene insoluble fraction is desirable only to a point, as extremely low isotacticity values will compromise the strength of the web in stretching. These characteristics are not easily obtained and depend on the catalyst system employed for the production of the polypropylene.
Failure to obtain the proper characteristics in the core material may result in various processing problems, including but not limited to, web breakage during orientation of the cast sheets, inconsistent film thickness and a narrow range of processing temperatures.
Different manufacturers have various theories about which properties in a propylene polymer are important in determining processability and film properties for core material. Some manufacturers claim that the stretchability of the film depends linearly on the amount and “quality” of stereoblock. Other manufacturers consider the amount of meso-racemic (mr) triad in the polymer to be critical.
U.S. Pat. No. 6,201,079, to Streeky, et al. discloses that for optimal fabrication performance in oriented films, a propylene polymer resin should have a specific combination of extractables and soluble components at a specific tacticity, as measured by the percent meso pentad (mmmm) determined by 13C NMR, to produce a broad processing window. According to the disclosure of U.S. Pat. No. 6,201,079 the general goal is to lower the isotacticity of the total polymer, measured as % mmmm, for a given content of decalin solubles in the resin.
A study was performed focusing on obtaining a better understanding of the structure-processing-properties relationships in BOPP films. It is known that the polypropylene should have at least a certain amount of atactic polypropylene, i.e., xylene solubles, to have good processability. However, the amount xylene solubles alone does not guarantee the processability of a polypropylene. Through this study it was determined that by balancing two characteristics of a propylene homopolymer, namely the quantity of xylene solubles and the average meso sequence length, Nm, of the xylene insoluble fraction, consistent processability of propylene homopolymer as a core material in BOPP film manufacture can be obtained.